The invention relates to a magnetic layer on a substrate for storing information in the form of a fixed, two-dimensional array of magnetic domains, which layer can be magnetized in either of two opposite directions, the walls of the domains being fixed by local gradients in the magnetic properties of the magnetic layer.
Such a magnetic layer is used especially for magneto-optical memories as described, for example, in IEEE Transactions on Magnetics, Vol. Mag-11 No. 5, Sept. 1975, pp. 1097-1102. The magnetic layer has a magnetization perpendicular to the surface thereof and the stored binary information is given by the direction of the magnetization. In order to be able to store a large amount of information (for example 10.sup.6 bits/cm.sup.2), the layer must be subdivided into separate information areas which preferably have the form of squares or circles arranged in rows and columns. The subdivision must be carried out so that the magnetic domain of an information area cannot expand into any adjacent information areas. This has been obtained, in known magnetic layers, by etching the layers down to the substrate near the information area boundaries. In this manner a large number of magnetic islands are formed on the substrate, one domain corresponding to one island, where the boundary of the domain is the edge of the island. As a result of this process a local gradient in the magnetic properties is formed at the junction between the magnetic material of the layer at the edge of the island and a non-magnetic material, generally air.
During the manufacture of the known magnetic layers an island-forming pattern of grooves is etched in the layer by using a mask which covers the layer in the region of the information areas and does not cover it at the area of the grooves to be etched away. In connection with the thickness of the magnetic layer required for a sufficiently large signal-to-noise ratio, the grooves must have a certain minimum width. On the other hand it is endeavored to minimize the size of the individual domains and the spaces between them so as to obtain a maximum storage density, that is a maximum amount of information per unit surface area. However, all the etching methods known so far provide only a restricted resolving power and a restricted steepness of the sides of the magnetic islands with a given etching depth, so that the need for a better structuring method exists. In addition, it would be advantageous to provide an improved structuring method which the formation of arbitrary two-dimensional domain patterns. This is an advantage when the magnetic layer is used in a system operating with multiple laser beams.